NOx emissions from diesel engines are an environmental problem. Several countries, including the United States, have long had regulations pending that will limit NOx emissions from trucks and other diesel-powered vehicles. Manufacturers and researchers have put considerable effort toward meeting those regulations.
In gasoline powered vehicles that use stoichiometric fuel-air mixtures, three-way catalysts have been shown to control NOx emissions. In diesel-powered vehicles, which use compression ignition, the exhaust is generally too oxygen-rich for three-way catalysts to be effective.
Several solutions have been proposed for controlling NOx emissions from diesel-powered vehicles. One set of approaches focus on the engine. Techniques such as exhaust gas recirculation and partially homogenizing fuel-air mixtures are helpful, but these techniques alone will not eliminate NOx emissions. Another set of approaches remove NOx from the vehicle exhaust. These include the use of lean-burn NOx catalysts, selective catalytic reduction (SCR) catalysts, and lean NOx traps (LNTs).
Lean-burn NOx catalysts promote the reduction of NOx under oxygen-rich conditions. Reduction of NOx in an oxidizing atmosphere is difficult. It has proven challenging to find a lean-burn NOx catalyst that has the required activity, durability, and operating temperature range. A reductant such as diesel fuel must be steadily supplied to the exhaust for lean NOx reduction, introducing a fuel economy penalty of 3% or more. Currently, peak NOx conversion efficiencies for lean-burn NOx catalysts are unacceptably low.
SCR generally refers to selective catalytic reduction of NOx by ammonia. The reaction takes place even in an oxidizing environment. The NOx can be temporarily stored in an adsorbent or ammonia can be fed continuously into the exhaust. SCR can achieve high levels of NOx reduction, but there is a disadvantage in the lack of infrastructure for distributing ammonia or a suitable precursor. Another concern relates to the possible release of ammonia into the environment.
LNTs are devices that adsorb NOx under lean exhaust conditions and reduce and release the adsorbed NOx under rich conditions. An LNT generally includes a NOx adsorbent and a catalyst. The adsorbent is typically an alkaline earth compound, such as BaCO3 and the catalyst is typically a combination of precious metals including Pt and Rh. In lean exhaust, the catalyst speeds reactions that lead to NOx adsorption. In a reducing environment, the catalyst speeds reactions by which hydrocarbon reductants are converted to more active species, speeds the water-gas shift reaction, which produces more active hydrogen from less active CO, and speeds reactions by which adsorbed NOx is reduced and desorbed. In a typical operating protocol, a reducing environment will be created within the exhaust from time-to-time to regenerate (denitrate) the LNT.
Regeneration to remove accumulated NOx may be referred to as denitration in order to distinguish desulfation, which is carried out much less frequently. A reducing environment can be created in several ways. One approach uses the engine to create a rich exhaust-reductant mixture. For example, the engine can inject extra fuel into the exhaust within one or more cylinders prior to expelling the exhaust. A reducing environment can also be created by injecting a reductant into lean exhaust downstream from the engine. In either case, a portion of the reductant is generally expended reacting with and consuming excess oxygen in the exhaust. To lessen the amount of excess oxygen and reduce the amount of reductant expended consuming excess oxygen, the engine may be throttled, although such throttling may have an adverse effect on the performance of some engines.
Reductant can consume excess oxygen by either combustion or reforming reactions. Typically, the reactions take place upstream of the LNT over an oxidation catalyst or in a fuel reformer. The reductant can also be oxidized directly in the LNT, but this tends to result in faster thermal aging. U.S. Pat. Pub. No. 2004/0050037 (hereinafter “the '037 publication”) describes an exhaust system with a fuel reformer placed in an exhaust line upstream from an LNT. The reformer includes both oxidation and steam reforming catalysts. The reformer removes excess oxygen from the exhaust and converts a portion of the diesel fuel reductant into more reactive reformate.
In addition to accumulating NOx, LNTs accumulate SOx. SOx is the combustion product of sulfur present in ordinarily fuel. Even with reduced sulfur fuels, the amount of SOx produced by combustion is significant. SOx adsorbs more strongly than NOx and necessitates a more stringent, though less frequent, regeneration. Desulfation requires elevated temperatures as well as a reducing atmosphere.
It is desirable to control the LNT temperature closely during desulfation. If the LNT temperature is too low, desulfation takes an excessive length of time, resulting in a high fuel penalty. If the LNT temperature is to high, the catalyst undergoes irreversible deactivation.
The temperature of the exhaust can be elevated by engine measures, particularly in the case of a lean-burn gasoline engine, however, at least in the case of a diesel engine, it is often necessary to provide additional heat to the LNT. Typically, this heat is provided through the same means used to remove excess oxygen from the exhaust. In the '037 publication, heat produced by the upstream fuel reformer is used to heat the downstream LNT to desulfations temperatures. The LNT temperature is then controlled through the fuel reformer temperature. The fuel reformer also provides syn gas, which is more active for desulfation than diesel fuel.
While producing syn gas, the fuel reformer may heat uncontrollably, particularly if the exhaust oxygen concentration is 8% or higher. The '037 publication addresses this problem by pulsing the fuel injection. During rich phases, the fuel reformer produces syn gas and heat. During lean phases, the fuel reformer is allowed to cool. By alternating between lean and rich phases, the fuel reformer temperature can be maintained within a narrow range.
Alternating between lean and rich phases during desulfation is also described in U.S. Pat. No. 6,530,216 (“the '216 patent”), but for an entirely different reason. According to the '216 patent, SOx is released as H2S if the reductant concentration is high and the rich phases are long. H2S release is undesirable due to its powerful and unpleasant odor. Low reductant concentrations are undesirable because desulfation is prolonged. By pulsing the fuel injection to limit the rich phases to 2 to 10 seconds each, high reductant concentrations can be used while releasing SOx as SO2 rather than H2S.
In spite of advances, there continues to be a long felt need for an affordable and reliable diesel power generation system that is durable, has a manageable operating cost (including fuel penalty), and limits NOx emissions to a satisfactory extent in the sense of meeting U.S. Environmental Protection Agency (EPA) regulations effective in 2010 and other such regulations.